PROJECT SUMMARY/ABSTRACT Regular physical activity or exercise (EX) is an effective strategy for primary breast cancer prevention. Numerous studies also indicate that EX significantly decreases the risk of recurrence, and improves survival in breast cancer patients suggesting an important role for EX in the long-term health of breast cancer survivors. To date, the biological mechanism(s) underlying the relationship between EX and reduced recurrence and improved survival from breast cancer are not well understood. Our preliminary data using the immunocompetent, syngeneic 4T1.2 metastatic mammary tumor model indicate that moderate EX in weight stable mice results in reduced primary tumor growth and metastatic burden. This occurs concurrently with an augmentation of T cell function and a reduction in the accumulation of myeloid derived suppressor cells, an immune suppressive cell important in the metastatic process. Thus, EX and/or weight maintenance (WM) may reduce recurrence and increase survival in breast cancer patients by preventing or delaying metastatic progression. In the current application, we will determine if the protective effect of our intervention on tumor growth and metastases is due largely to changes in energy balance (e.g. the prevention of weight gain) or to direct effect of EX (independent of changes in body weight) on tumor outcomes and metastases. BALB/c mice will be randomized to EX or sedentary control (SED) groups and given access to running wheel or control cages, respectively, for 8 weeks prior to the injection of 5X104 4T1.2 cells, a metastatic tumor cell line, into the 4th mammary gland. One cohort of SED mice will be fed ad libitum (SED/AL) and a cohort of EX mice will be weight matched to the SED/AL group. Additionally, one cohort of EX mice will be energy-restricted by 10% to achieve an EX/ER group that weighs less than the SED/AL group. A second cohort of SED mice will be weight matched to the EX/ER group. In addition to quantifying primary tumor growth and metastatic burden (Aim 1), we will determine if EX or WM alters the pro- vs. anti-tumor immune environment of the host (Aim 2), and/or the metabolic and inflammatory milieu (Aim 3); and which of these mediators are related to tumor growth and metastatic progression. To date, no studies have addressed these questions in a relevant breast cancer metastases model. Results from the studies proposed will shed critical insight on the mechanisms by which EX and/or WM may be exerting a secondary and tertiary cancer preventive effect, and move the field forward by honing in on specific biological pathways, and identify targets for future experiments. Identification of key mechanisms linking EX and/or WM to reduced metastatic burden may ultimately enable us to design more effective preventative and therapeutic strategies that include exercise and weight maintenance interventions.